# Wht usually goes wrong: my experience with MOPAC

```      Seeing some messages on minimization problems with negative
frequencies of late here, I thought I'd add some of my experiences.
definition, or worse, forcing an artificial symmetry either through
bad z-matrix definition or through actual symmetry constraints, causes
one to minimize to a hilltop or hillside while one's gradients happily
descend to near zero. At least with MOPAC 6, and perhaps with other
MOPAC's, gradients are determined from the z-matrix definition of
coordinates, and if a particularly shallow twist is defined along the
coordinates, while there is an (undefined) twist that is steep, you
could spend a lot of computation hours in small changes in geometry
that ignore the larger gradients present and fail to completely
minimize the molecule. It would probably be worse for
graphical-interfaces where you have little to no control over such
failures. Ways to resolve: use a different minimizer, or even try XYZ
to minimize in cartesian space. Or, set the gradient to minimize
further than you currently are. Or: try a different program. Sometimes
the cure is to see if you can repeat it on another machine or program.
2. failure to minimize: sometimes you think you've minimized enough,
but you do something inadvertant that messes that up. Most programs
are smart enough to make sure that you have not been subject to
bizarre rounding errors, but sometimes it makes sense to double check
everything. Bond angles near 180 degrees can be the biggest headache,
and it is conceivable that a rounding error might cause large geometry
changes. Ways to resolve: check the geometry after minimization, and
before and after frequency calcs. Also, make sure you haven't invoked
a series of keywords that cancel one another or set different
minimization defaults. Another big bad example (in ab-initio, not
mopac) is minimizing in one basis set, and then doing a freq calc in
another. There's nothing stopping a novice in principle from
minimizing in MM2, ie, in Hyperchem, then using a single point
calculation to print frequencies in AM1. That's probably a big error.
I realize many, if not most, of the people here are not novices. But
perhaps we should, in true internet style, have a faq on such things!
-matt
```